In recent years there has been a proliferation of communication facilities involving many remote stations and terminals working together with data processors in a network. Generally, such network systems involve a host processor working with a main memory to form a central processing unit, or even a plurality of such central processing units, whereby digitized message data can be transmitted from one station or terminal to another station or terminal within the system, but which, of course, the transmission must be routed, controlled and organized to accomplished the message transfer in an orderly and accurate fashion.
In the prior art, one example of the handling of digital communications between remote terminals and a central processor is seen in U.S. Pat. No. 3,618,037, inventor James E. Wollum and entitled "Digital Data Communication Multiple Line Control," which patent is also assigned to the assignee of the presently described invention.
In the field of data communications each data transmission line is connected to a "line adapter" which interfaces the data communications line into the system network. These line adapters may be associated together in a group and called an Adapter Cluster or, that is to say a group or cluster of adapters physically located within one unit. Each line adapter is specially designed to operate to suit the characteristics of a particular type of remote terminal or station. The line adapter has to take into account account factors such as the type of characters transmitted, the coding type of characters, the type of parity that is used, whether transmission is synchronous or asynchronous, the data rate or speed of transmission permissible, and so on, in order to provide that the terminal station connected at the other end of the transmission line will receive the proper type of signals.
Efforts are continuously being made to increase throughput, i.e., the number of message bits that can accurately be transmitted per unit time while minimizing the cost of equipment and facilities for accomplishing this. However, there must also be flexibility, in that provision must be made for wide band high speed transmission line for high speed transmission of data, in addition to low to medium speed transmission lines which are commonly used since they are cheaper in cost. Further, the accessibility of message data stored in memory must be speedily available in order to obviate delays and increase throughput, and the desirability of concurrent overhead control operations to reduce delays has been recognized.
The field of this invention pertains to data processing equipment which is intended for use with a wide variety of remotely located terminal devices. It has become very desirable to incorporate a data processing system into a network for transmission of data over long distances. The terminal devices involved will generally convert the data from a humanly readable form into binary digital form and transmit this data over wires or microwave relay systems. The terminal devices operate under and generate a wide variety of messages code sets, character lengths, bit rates, message formats, communication line disciplines and modes of transmission which present considerable problems to the designer of data communication equipment. The data communications equipment must be able to interface with a wide variety of different types of these terminal devices and should be flexible enough that additional devices can be added or that the terminal devices already used can be changed according to customer preference.
Many of the past and presently existing data communication systems are categorized by those systems which are designed with fixed hardware and are intended only with a specific type of terminal device. This may be economical but is not particularly flexible; other systems have been designed in a modular form to provide options for each of the modules to provide compatibility with certain types of terminal equipment. Because of the differences required among different line disciplines and different types of terminal requirements, it is not usually possible to design a common logic system to perform control functions to cover each of the variety of types of terminals. Among the difficulties involved is that of providing a comprehensive software package to service different configurations and in which the configurations may be desired to be changed from time to time. Thus, in the economics of time and hardware it has often been found necessary to limit the software to one particular type of data communication lines and terminal stations in the system.
With the development of integrated circuits and mini and microcomputers, it is now possible to provide hardware and software of great flexibility in order to handle systems which may have many possible configurations and newly desired configurations in the future. Often it was necessary that a particular program or subroutine be provided for each type of terminal device connected to the system and when new terminals were added to the system, a new subroutine was provided. This activity, however, lead to considerable expense, in addition to eating up long periods of time within the processor.
The present invention overcomes many of the earlier limitations and provides faster throughput of data transfers while permitting reconfigurability and also adaptability to various types of transmission lines and terminal equipment characteristics.
The use of individualized adapters as part of data communications systems in which the adapters are connected to remote terminals, such as teletype units, video terminals, etc., via telephone lines, has required a great amount of overhead operations to be imposed upon either a local processor, such as a data communications processor or upon a main host processor if no local data communications processor is available.
For example, different types of remote terminals used in a data communications system will require different codes such as ASCII or EBCDIC; also, the various remote terminals operate at different speeds and thus require specialized timing cycles; further the byte sizes of words used in different remote terminals will vary according to the type and make of terminal to further complicate the communication discipline involved; further, different terminals may require different protocols and recognition of specialized control characters.
Thus, in order that orderly data communications take place between a set of remote terminals and a central station, various types of arrangements must be made so that the variety of terminals may be received and standardized into formats which are handleable by the central station. Further, since the remote terminals operate on a relatively slow basis, generally transmitting or receiving one byte of information during a transmission cycle and then requesting an interrupt of a processor for further instructions, it is extremely burdensome and time consuming on the part of the local data communications processor to monitor, control and regulate the data transmissions for each individual byte (received or transmitted) for each of the individual remote terminals. The various requirements of, for example, eight remote terminals require a tremendous need for overhead operations on the part of the data communications processor which results in overall data transfer operations being slowed down, since the data communications processor is not readily avialable when it is occupied in handling the data communications transfer requirements of a variety of different remote terminals.